Microstructural evolution of mechanically deformed polycrystalline silicon for kerfless photovoltaics

Silicon wafers for photovoltaics could be produced without kerf loss by rolling, provided sufficient control of defects such as dislocations can be achieved. A study using mainly high resolution electron backscatter diffraction (HR‐EBSD) of the microstructural evolution of Siemens polycrystalline silicon feedstock during a series of processes designed to mimic high temperature rolling is reported here. The starting material is heavily textured and annealing at 1400 °C results in 90% recrystallization and a reduction in average geometrically necessary dislocation (GND) density from >1014 to 1013 m−2. Subsequent compression at 1150 °C – analogous to rolling – produce sub‐grain boundaries seen as continuous curved high GND content linear features spanning grain interiors. Post‐deformation annealing at 1400 °C facilitates a secondary recrystallization process, resulting in large grains typically of 100 μm diameter. HR‐EBSD gives the final average GND density in as 3.2 × 1012 m−2. This value is considerably higher than the dislocation density of 5 × 1010 m−2 from etch pit counting, so the discrepancy is investigated by direct comparison of GND maps and etch pit patterns. The GND map from HR‐EBSD gives erroneously high values at the method's noise floor (≈1012 m−2) in regions with low dislocation densities

Solar Wind Sodium and Potassium Abundance Analysis in Genesis Diamond-on-Silicon and Silicon Bulk Solar Wind Collectors, and How Hydration Affects the Microtexture of Olivine Phase Transformation at 18 GPa

abstract: The present work covers two distinct microanalytical studies that address issues in planetary materials: (1) Genesis Na and K solar wind (SW) measurements, and (2) the effect of water on high-pressure olivine phase transformations. NASA’s Genesis mission collected SW samples for terrestrial analysis to create a baseline of solar chemical abundances based on direct measurement of solar material. Traditionally, solar abundances are estimated using spectroscopic or meteoritic data. This study measured bulk SW Na and K in two different Genesis SW collector materials (diamond-like carbon (DlC) and silicon) for comparison with these other solar references. Novel techniques were developed for Genesis DlC analysis. Solar wind Na fluence measurements derived from backside depth profiling are generally lower in DlC than Si, despite the use of internal standards. Nevertheless, relative to Mg, the average SW Na and K abundances measured in Genesis wafers are in agreement with solar photospheric and CI chondrite abundances, and with other SW elements with low first ionization potential (within error). The average Genesis SW Na and K fluences are 1.01e11 (+9e09, -2e10) atoms/cm2 and 5.1e09 (+8e08, -8e08) atoms/cm2, respectively. The errors reflect average systematic errors. Results have implications for (1) SW formation models, (2) cosmochemistry based on solar material rather than photospheric measurements or meteorites, and (3) the accurate measurement of solar wind ion abundances in Genesis collectors, particularly DlC and Si. Deep focus earthquakes have been attributed to rapid transformation of metastable olivine within the mantle transition zone (MTZ). However, the presence of H2O acts to overcome metastability, promoting phase transformation in olivine, so olivine must be relatively anhydrous (<75 ppmw) to remain metastable to depth. A microtextural analysis of olivine phase transformation products was conducted to test the feasibility for subducting olivine to persist metastably to the MTZ. Transformation (as intracrystalline or rim nucleation) shifts from ringwoodite to ringwoodite-wadsleyite nucleation with decreasing H2O content within olivine grains. To provide accurate predictions for olivine metastability at depth, olivine transformation models must reflect how changing H2O distributions lead to complex changes in strain and reaction rates within different parts of a transforming olivine grain.Dissertation/ThesisDoctoral Dissertation Geological Sciences 201

Development of CCDs for REXIS on OSIRIS-REx

The Regolith x-ray Imaging Spectrometer (REXIS) is a coded-aperture soft x-ray imaging instrument on the OSIRIS-REx spacecraft to be launched in 2016. The spacecraft will fly to and orbit the near-Earth asteroid Bennu, while REXIS maps the elemental distribution on the asteroid using x-ray fluorescence. The detector consists of a 2×2 array of backilluminated 1k×1k frame transfer CCDs with a flight heritage to Suzaku and Chandra. The back surface has a thin p[superscript +]-doped layer deposited by molecular-beam epitaxy (MBE) for maximum quantum efficiency and energy resolution at low x-ray energies. The CCDs also feature an integrated optical-blocking filter (OBF) to suppress visible and near-infrared light. The OBF is an aluminum film deposited directly on the CCD back surface and is mechanically more robust and less absorptive of x-rays than the conventional free-standing aluminum-coated polymer films. The CCDs have charge transfer inefficiencies of less than 10[superscript -6], and dark current of 1e-/pixel/second at the REXIS operating temperature of –60 °C. The resulting spectral resolution is 115 eV at 2 KeV. The extinction ratio of the filter is ~10[superscript 12] at 625 nm.United States. National Aeronautics and Space Administration. Strategic Astrophysics Technology Program (Grant NNX12AF22G)United States. National Aeronautics and Space Administration (Contract NNG12FD70C)United States. National Aeronautics and Space Administration (IPR NNG12FC01I)United States. National Aeronautics and Space Administration. Strategic Astrophysics Technology Program (IPR NNH12AU04I)United States. Air Force (Contract FA8721-05-C-0002

Function-led design of multifunctional stimuli-responsive superhydrophobic surface based on hierarchical graphene-titania nanocoating

Multifunctional smart superhydrophobic surface with full-spectrum tunable wettability control is fabricated through the self-assembly of the graphene and titania nanofilm double-layer coating. Advanced microfluidic manipulative functions, including directional water transport, adhesion & spreading controls, droplet storage & transfer, and droplet sensing array, can be readily realized on this smart surface. An in-depth mechanism study regarding the underlying secrets of the tunable wettability and the UV-induced superhydrophilic conversion of anatase titania are also presented

Experimental and theoretical evaluation of in-depth damage distribution in sawn silicon wafers

As-sawn silicon wafers have surface damage that needs to be removed before any further processing into solar cells. This damage distribution can vary with cutting parameters such as wire size, slurry particle/diamond grit size, and wire usage. To date, there is no simple way to measure the degree of damage, damage depth, and damage distribution. But, this information is needed by the wafer manufacturers as well as solar cell manufacturers. A technique based on sequential etching of silicon wafers and minority carrier lifetime (τeff) measurements is used to determine damage depth. In this technique, samples are sequentially etched to remove thin layers from each surface and minority carrier lifetime is measured after each etch step. Lifetime increases after each layer of damage is removed and reaches a plateau once the damage is totally removed. The thickness-removed at which the lifetime reaches a peak value corresponds to the damage depth. An accurate measurement of τeff requires corrections to optical reflection, and transmission from silicon wafers to account for changes in the surface morphology and in the wafer thickness. This technique also allows the in-depth distribution of the damage to be quantified in terms of surface recombination velocity (SRV). Although this method is routinely used at the National Renewable Energy Laboratory to measure damage depth, determination of damage distribution from these data requires an accurate model that coverts the minority carrier lifetime data into carrier recombination distribution. Continuity equation for excess minority carrier density (Δn) is solved for the material of interest (silicon wafer with surface damage layer), and carrier concentration is integrated and normalized to match the normalized lifetime vs thickness removed curve. A simplified model for determining the recombination distribution within a wafer having surface damage is presented. Potential improvements for this model are discussed

Biomimetic nanostructured surfaces for antireflection in photovoltaics

A key consideration in the design of any solar cell is the reduction of reflectance from the top surface. Traditional thin film antireflection schemes are being challenged by new techniques that involve texturing on the subwavelength scale to form ‘moth-eye’ arrays, so called because they are inspired by Nature’s answer to unwanted reflections, the arrays of pillars found on the eyes and wings of some species of moth. In this work, a new method is presented for the optimization of thin film coatings that accounts for the angular and spectral variations in incident solar radiation from sunrise to sunset. This approach is then extended to silicon moth-eye arrays to assess how effectively these surfaces can provide antireflection for silicon solar cells over a full day. The reflectance spectra of moth-eye surfaces are found to depend on the period of the arrays and the height and shape of the pillars, and consequently these parameters can be optimized for the solar spectrum. Simulations predict that replacing an optimized double layer thin film coating with a moth-eye array could increase the full day cell performance by 2% for a laboratory cell and 3% for an encapsulated cell. Compared to a perfectly transmitting interface, this corresponds to losses in short circuit current of only 5.3% and 0.6% for a laboratory and an encapsulated cell, respectively. Furthermore, fabrication of silicon moth-eye arrays by electron beam lithography and dry etching leads to predicted percentage losses at peak irradiance, compared to an ideal antireflective surface, of only 1%. The potentially more scalable technique of nanoimprint lithography is also used to fabricate antireflective moth-eye arrays in silicon, over areas as large as 1 cm2, demonstrating great potential for stealth and antiglare applications in addition to photovoltaics

SURFACE ETCHING TECHNOLOGIES FOR MONOCRYSTALLINE SILICON WAFER SOLAR CELLS

Ph.DDOCTOR OF PHILOSOPH

Advanced Luminescence-based characterisation of silicon wafer solar cells

Ph.DDOCTOR OF PHILOSOPH

Metrology and Characterisation of Defects in Thin-Film Barrier Layers Employed in Flexible Photovoltaic Modules

Flexible thin-film photovoltaic (PV) modules based on copper indium gallium selenide (CIGS) materials are one of the most recent developments in the renewable energy field, and the latest films have efficiencies at or beyond the level of Si-based rigid PV modules. Whilst these films offer significant advantages in terms of mass and the possibility of building-integrated photovoltaic (BIPV) applications, they are at present highly susceptible to long term environmental degradation as a result of water vapour transmission through the protective encapsulation layer to the active (absorber) layer. To maintain the PV module flexibility and to reduce or eliminate the water vapour permeability, the PV encapsulation includes a barrier layer of amorphous aluminium oxide (Al2O3) material of a few nanometres thickness deposited on a planarised polyethylene naphthalate (PEN) substrate. The highly conformal barrier layer of the Al2O3 is produced by atomic layer deposition (ALD) methods using roll-to-roll (R2R) technology. Nevertheless, water vapour permeation is still facilitated by the presence of micro and nano-scale defects generated during the deposition processes of the barrier material, which results in decreased cell efficiency and reduced unit longevity. The state of the art surface metrology technologies including: optical microscopy, white light scanning interferometry (WLSI), atomic force microscopy (AFM) and scanning electron microscopy (SEM) were extensively deployed in this project as offline surface characterisation methods to characterise the water vapour barrier layer defects, which are postulated to be directly responsible for the water vapour ingress. Areal surface texture parameters analysis based on wolf pruning, area pruning and segmentation analysis methods as defined in ISO 25178-2; allow the efficient separation of small insignificant defects from significant defects. The presence of both large and small defects is then correlated with the barrier films functionality as measured on typical sets of Al2O3 ALD films using a standard MOCON® (quantitative gas permeation) test. The investigation results of the initial analysis finishes by drawing conclusions based on the analysis of the water vapour transmission rate (WVTR), defects size, density and distribution, where it is confirmed that small numbers of large defects have more influence on the deterioration of the barrier films functionality than large numbers of small defects. This result was then used to provide the basis for developing a roll-to-roll in process metrology device for quality control of flexible PV barrier films. Furthermore, a theoretical model approach was developed in this thesis based on the water vapour diffusion theory to determine the cut- off level between large significant defects and small insignificant defects. The results of the model would seem to reveal that, in order to build up in process, non-contact optical defect detection system for R2R barrier films, the critical spatial resolution required for defect detection need not be less than 3 μm laterally and 3Sq nm (Sq= root mean square surface roughness deviation of non-defective sample area) per field of view (FOV) vertically. Any defect that has dimensions less than this appears to have a significantly lower effect on the PV barrier properties and functionality. In this study, the surface topography analysis results and the theoretical model approach outcomes, both provide the basis for developing a R2R in process metrology device for PV barrier films defect detection. Eventually, the work in this thesis reports on the deployment of new (novel) in-line interferometric optical sensors based on wavelength scanning interferometry (WSI) designed to measure and catalogue the PV barrier films defects where they are present. The sensors have built-in environmental vibration compensation and are being deployed on a demonstrator system at a R2R production facility in the UK